Five Things That Could Spook Stock Investors This October

3. Technical Levels

Many analysts say the market's momentum has been based strongly on technical benchmarks that have been eclipsed after the market reached a strongly oversold position in March.

Similarly, some are now starting to wonder if the market hasn't reached a resistance level from which it will correct following the rally.

"Trading has picked up. That's also something that makes people nervous," Krosby says. "Once the day traders get in there, they typically come in at the end of a momentum-driven rally."

"If we have a consolidation, which is the most minor form of a pullback, that's one thing. If it's something larger than a consolidation, those riskier assets are going to sell off and sell off dramatically."

Questions about the technical aspect of the rally have heightened as the S&P has shown resistance at the 1065 level, pulling back once it passed that area.

That's not necessarily a long-term bearish sign for the market, but could signal an impending correction.

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"Support remains 1014-1000 with key support at the August low of 978.50," BofA Merrill Lynch Global Research analyst MaryAnn Bartels wrote in a note to clients. "We maintain that the risk of a 15-20% correction is rising within the context of a base-building process and that the major area of resistance at 1200-1325 can be tested in 2010."

4. Financials

After 18 months at the center of the market firestorm, beaten-down financials have been a major player in the stocks rally.

Now, with many of the industry's biggest names seemingly back on their feet, their performance will be watched closely for clues about the broader market.

"We're watching the financial stocks very closely," Krosby says. "We don't want to see the best of breed sell off."

Analysts have been watching the yield curve—specifically, the gap between yields on the 10-year and 2-year Treasury notes—and drawing caution about what it might portend for the sector. A wide spread generally means good things for the group, but a narrowing spread, as has been happening lately, could be a sign of trouble.

5. Everything Else

Geopolitics, commodity prices, dollar moves—they're all in the mix as well as the world endures a tumultuous time of saber rattling in the Mideast, declining faith in the US currency and a health care battle at home.

"The news is still really bad, so I'm looking for a reaction in October," Boyle says. "However, there's enough money on the sidelines and these big hedge-funds are throwing these programmed buy trades in. We see a lot of business controlled by programmed trades."

Of course, that can cut both ways, as the market has found out the hard way in the 18 months preceding the March rally.

Fear is still a strong ingredient in this market environment.

"I'm in the skeptic crowd," Art Cashin, director of floor operations at UBS, told CNBC. "I think it's going to be tough for the economy to live up to the hope and hype that we've seen in some of these stocks."

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